Morphogenic proteins ("morphogens") are well known and described in the art. See, for example, U.S. Pat. Nos. 4,968,590; 5,011,691; 5,018,753; PCT US92/01968 and PCT US92/07432; as well as various articles published in the scientific literature, including Ozkaynak et al. (1992) J. Biol. Chem. 267:25220-25227 and Ozkaynak et al. (1991) Biochem. Biophys. Res. Comm. 179:116-123. The art has described how to isolate morphogenic proteins from bone, how to identify genes encoding these proteins and how to express them using recombinant DNA technology. The morphogenic proteins are capable of inducing endochondral bone formation and other tissue formation in a mammal when they are properly folded, dimerized and disulfide bonded to produce a dimeric species having the appropriate three dimensional conformation. The proteins have utility in therapeutic applications, either by direct or systemic administration. Where bone induction is desired, for example, the morphogen typically is provided to the desired site for bone formation in a mammal in association with a suitable matrix having the appropriate conformation to allow the infiltration, proliferation and differentiation of migrating progenitor cells. The morphogenic protein adsorbed to the surfaces of a suitable matrix is generally referred to in the art as an osteogenic device. The proteins can be isolated from bone or, preferably, the gene encoding the protein is produced recombinantly in a suitable host cell.
The morphogen precursor polypeptide chains share a common structural motif, including a N-terminal signal sequence and pro region, both of which are cleaved to produce a mature sequence, capable of disulfide bonding and comprising an N-terminal extension and a C-terminal domain whose amino acid sequence is highly conserved among members of the family. In their mature dimeric forms, the morphogens typically are fairly insoluble under physiological conditions. Increasing the solubility of these proteins has significant medical utility as it would enhance systemic administration of morphogens as therapeutics. Various carrier proteins, including serum albumin and casein are known to increase the solubility of morphogens (see, for example, PCT US92/07432). PCT US92/05309 (WO 93/00050) discusses the use of various solubilizing agents, including various amino acids and methyl esters thereof, as well as guanidine, sodium chloride and heparin, to increase the solubility of mature dimeric BMP2.
Improved methods for the recombinant expression of morphogenic proteins is an ongoing effort in the art. It is an object of this invention to provide an improvement in the methods for producing and purifying morphogenic proteins having high specific activity, and for formulating compositions and osteogenic devices comprising these proteins. Another object is to provide soluble forms of morphogenic proteins consisting essentially of amino acid sequences derived from morphogenic proteins. Another object is to provide formulations which stabilize the soluble complex of morphogenic proteins. Still another object is to provide means for distinguishing between soluble forms of the protein and the mature morphogenic species, to provide means for quantitating the amounts of these proteins in a fluid, including a body fluid, such as serum, cerebro-sprinal fluid or peritoneal fluid, and to provide polyclonal and monoclonal antibodies capable of distinguishing between these various species.
Another object is to provide antibodies and biological diagnostic assays for monitoring the concentration of morphogens and endogenous anti-morphogen antibodies present in a body fluid and to provide assays for detecting fluctuations in the concentrations of these proteins in a body fluid. U.S. Pat. No. 4,857,456 and Urist et al. (1984) Proc. Soc. Exp. Biol. Med. 176:472-475 describe a serum assay for detecting a protein purported to be a bone morphogenetic protein. The protein is not a member of the morphogen family of proteins described herein, differing in molecular weight, structural characteristics and solubility from these proteins.